secant piles secant pile walls are formed by...

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a Division of Brierley Associates, Inc.

SECANT PILESo Secant pile walls are formed by

constructing a series of overlapping concrete-filled drill holes to form a continuous, relatively watertight wall.

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CONSTRUCTION PROCESSo Alternate primary

(initial) and secondary (closure) piles.

o Primary piles are installed first followed by the secondary piles. Secondary piles are “cut in” to the primary piles.

o Piles can be filled with either structural or lean concrete.

CONSTRUCTION PROCESSo Drilling Methods

• Kelly drilling: Allows a range of soil and rock tooling to be utilized within a cased hole as the hole is advanced.the hole is advanced.

• Sectional heavy wall drill casing, advanced concurrently with the drill tool maintains hole stability and stiffens the drill string.

• Top drive rotary crawler drills ideally suited for drills ideally suited for secant pile drilling.

• Pile diameters: 24”-48”.• Oscillator attachment can

be used to assist casing advance and extraction.

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REINFORCING OPTIONSo Typically secondary piles are reinforced. Much less common

to reinforce both the primary and secondary piles.

Wide Flange Insert Rebar Cage

SECANT PILE APPLICABILITYo Groundwater cut-offo Minimize ground losso Applicable for almost all geotechnical conditions including:o Applicable for almost all geotechnical conditions, including:

• Loose, cohesionless soil below the groundwater table• Soils with cobbles and boulders• Rock, including penetration into hard rock when required

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DESIGN CONSIDERATIONSo Designed and analyzed similar to other “continuous” wall

systems (e.g. sheet piles, deep soil mixing)o Reinforced piles are typically designed to act as beams p yp y g

spanning in the vertical direction.o Concrete between reinforced elements acts as “lagging.”o Cantilever or restrained (e.g. cross-lot bracing or tiebacks)

wall systems are feasible.o Can also function as unreinforced concrete ring structures with

the design based upon the minimum effective compression ring the design based upon the minimum effective compression ring thickness that can be developed by the overlapping piles with installation tolerances considered. Structurally-efficient.

INSTALLATION TOLERANCEo In order for a secant pile

wall to perform as intended overlap must be maintained between the primary and secondary piles.S ifi d il di t d o Specified pile diameter and spacing (which define the theoretical overlap) must allow for installation tolerance.

o Tolerances to consider:• Accuracy of pile

placement at the ground surface.

• Verticalit• Verticality.o Tighter tolerances than

typical drilled shaft work.o 80 feet has been

considered the practical limit of installation.

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TOLERANCE CONTROLo Ground surface location

• Survey alone (least accurate)• Guide trenchGuide trench• Template

Guide Trench Template

TOLERANCE CONTROLo Controlling verticality

• Verify plumbness of the drill string when starting and g gduring the drilling process.

• Use of stiff drill string.• Downhole survey techniques.

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DOWNHOLE SURVEY TECHNIQUESo Sonicaliper® (Loadtest)

Sonicaliper® Instrument Sonicaliper® Reading

DOWNHOLE SURVEY TECHNIQUESo Inclination Measuring Instrument (Bauer)

• Can be used to survey cased drillholes.• Similar to a large-scale inclinometer probeSimilar to a large-scale inclinometer probe.

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MORMON ISLAND AUXILIARY DAM KEY BLOCK

Project Overviewo MIAD is located about 20 miles northeast of Sacramento, California.o 4800-foot long, 110-foot high earth dam that helps impound the

American River to form Folsom Lake.American River to form Folsom Lake.o United States Bureau of Reclamation (BOR) has determined that

some of the foundation soils are susceptible to liquefaction during a large earthquake.

o BOR designed a 900-foot long by 55-foot wide “Key Block” downstream of the toe of the existing dam to mitigate potential problems resulting from liquefaction.

o Key Block excavation to be keyed into moderately weathered bedrock and backfilled with lean concrete and select fill.

o Owner: United States Bureau of Reclamation.o General Contractor: Shimmick Construction.


Approximate Locationof Key Block

Morman IslandAuxiliary Dam

Satellite View

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Contract Drawing - Section View

MORMON ISLAND AUXILIARY DAM KEY BLOCKGeotechnical Conditionso Soil Profile:

• Dredged alluvium: Sand and gravel with cobbles and occasional boulders. (Dredge tailings from old gold mining operations.) Material generally becomes siltier/finer-grained with depth.Material generally becomes siltier/finer grained with depth.

• Colluvium: Sporadic thin deposit of gravelly clay sitting on the bedrock in some borings.

• Soil mass in the area of the Key Block was previously treated with thousands of bottom feed stone columns.

• Bedrock: Amphibolite Schist. Typically intensely fractured with steeply dipping schistosity. Variably weathered. Hard where relatively fresh and very soft to soft where intensely weathered. Anticipated to have very low permeability Depth to bedrock Anticipated to have very low permeability. Depth to bedrock ranges from about 52 to 72 feet.

o Groundwater:• Typically within 5 feet of the ground surface.

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Test Section

Secant Pile Wall Plan Layout


Detail Plan at Test Section

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Typical Section


View Looking West

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Test Section Secant Pile Constructiono Bauer BG40 drill rig used to install piles.o Holes cased with sectional heavy wall casing. Casing oscillator

d t t t iused to extract casing.o Sonicaliper® downhole surveys were performed periodically

to confirm that verticality tolerance was being achieved.o Holes tremie-concreted. Wide flange inserts stabbed into

concrete-filled hole.o During excavation, observations of the as-built piles indicated

that installation tolerance was excellent. Seepage through secant pile joints was minimal.


Secant Pile Installation

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Secant Pile Installation


View from Top of Test Section Excavation

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Secant Piles Muck Box Being Used to Remove Soil


Bracing Levels 2, 3 and 4

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Test Section Shoring Performanceo Secant pile wall essentially watertight

with up to 60 feet of hydrostatic headhead.

o Excellent drilling tolerance achieved under difficult drilling conditions.

o Secant pile wall was very stiff. Inclinometers indicated less than about ½” of deflection.P f b d b f d

Test Section at Full Depth

o Pre-fabricated bracing frames and concrete packing proved to be very efficient.

NEW IRVINGTON TUNNEL – VARGAS SHAFT

Project Overviewo 3.5 mile long New Irvington Tunnel being constructed to provide

a seismically sound alternate to the existing tunnel which t S F i ’ t i th Si N d connects San Francisco’s water sources in the Sierra Nevada

and Alameda County to the Bay Area’s water supply systems.o Project includes a 115-foot deep, 41-foot diameter shaft to

create access to drive the 13-foot diameter tunnel in two directions.

o Owner: San Francisco Public Utilities Commission.o General Contractor: Southland Contracting/Tutor-Perini, JV.

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I-680

ApproximateApproximate ApproximateTunnel Centerline

ApproximateShaft Location

Satellite View


View of Shaft Collar Looking South

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Geotechnical Conditionso Soil Profile:

• Fill: 10 to 20 feet of loose to very dense silty sand with gravel and cobblesgravel and cobbles.

• Alluvium: about 20 feet of loose poorly graded sand with gravel and cobbles and medium stiff sandy lean clay.

• Bedrock: anticipated to be about 5 to 8 feet of very weak siltstone overlying weak to moderately strong, highly to moderately fractured sandstone

• Some shear zones in rock.o Groundwater:

• About 14 feet below ground surface.


Original Shaft Support Plano Secant pile compression ring penetrating a few feet into bedrock.o Rock dowels and shotcrete installed in a top-down manner in bedrock.

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Revised Shaft Support Plano During initial secant pile drilling it became evident that the top-down

rock dowel and shotcrete support would be very challenging to install due to the quality of the bedrock being encountered to a depth of

9 fabout 95 feet.o Shaft support design was revised to extend the secant pile

compression ring down to competent bedrock.o Unprecedented depth of a secant pile compression ring.o Concrete f’c = 3000 psi.o Specified tolerances to maintain 5 inches of overlap between

adjacent piles at a depth of 80 feet (in order to develop a 1.5-foot j p p ( pminimum thick effective compression ring).• Pile within 1 inch of theoretical location at ground surface. (Guide

trench provided.)• 0.5% verticality tolerance.


Secant Pile Plan Layout - Revised

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Shaft Section - Revised


Shaft Constructiono Bauer BG40 drill rig used to install piles.o Holes cased with sectional heavy wall casing to a depth of

approximately 100 feet. Uncased hole in hard rock below 100 feet.o Sonicaliper® downhole surveys were performed on every hole at

depths of 60 and 90 feet. Surveys generally indicated that the verticality tolerance was well within the 0.5% limit.

o Holes tremie-concreted. As-placed concrete volume about 25% more than theoretical hole volume.

o During excavation, observations of the as-built piles indicated that, for the most part, installation tolerance was excellent.p ,

o Weep holes provided through secant pile wall to relieve groundwater pressure in rock.

o Shaft successfully supported to a depth of 100 feet without any supplemental support.

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Shaft Excavated to 90 feet


Shaft at Full Depth

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CONCLUSIONS

o Drilling equipment and methods are now capable of achieving tolerances that allow secant piles to be used in geotechnical conditions and to depths previously considered to be unfeasible.considered to be unfeasible.

o Modern drill rigs and tooling allow cost-effective installation of secant piling excavation support systems suitable for depths up to 100 feet.

o Recently developed downhole survey techniques allow confirmation that critical drilling tolerances are met.

ACKNOWLEDGMENTS

MIAD Key Blocko Shimmick Construction

Trace Porter, Project Manager

o United States Bureau of Reclamation

Vargas Shafto Southland/Tutor-Perini, JV

Michael Cash, Project ManagerManager

o San Francisco Public Utilities Commission and their consultants a Division of Brierley Associates, Inc.

secant piles secant pile walls are formed by...

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